import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.SocketException;

public class UdpEchoServer {
    private DatagramSocket socket = null;
    public UdpEchoServer(int port) throws SocketException {
        socket = new DatagramSocket(port);
    }

    public void start() throws IOException {
        System.out.println("服务器启动!");
        while (true) {
            // 1) 读取请求并解析
            DatagramPacket requestPacket = new DatagramPacket(new byte[4096], 4096);
            socket.receive(requestPacket);
            // 为了方便在 java 代码中处理 (尤其是后面进行打印) 可以把上述数据报中的二进制数据, 拿出来, 构造成 String
            String request = new String(requestPacket.getData(), 0, requestPacket.getLength());
            // 2) 根据请求计算响应
            String response = this.process(request);
            // 3) 把响应写回到客户端
            DatagramPacket responsePacket = new DatagramPacket(response.getBytes(), 0, response.getBytes().length,
                    requestPacket.getSocketAddress());
            socket.send(responsePacket);

            System.out.printf("[%s:%d] req=%s, resp=%s\n", requestPacket.getAddress(), requestPacket.getPort(),
                    request, response);
        }
    }
    // 由于当前写的是 "回显服务器"
    public String process(String request) {
        return request;
    }
    public static void main(String[] args) throws IOException {
        UdpEchoServer server = new UdpEchoServer(9090);
        server.start();
    }
}